In fact, I'd say that Ireland's (directly elected) presidency has been one of the most notably successful, particularly in the post-1990 period when it's taken on more of a "cultural" rather than "political" focus.
A political head of state is not good imho, just ends up vetoing and doing politics instead of acting as a guardian of the constitution and institutions.
It's just a matter of "elect president from government's opposition and he's now emboldened to veto and block anything because he has popular mandate to do so".
Politicians electing the president seems a better option because they need to compromise on an individual that will allow any party to govern and be checked.
> In over 50 years not a single parliamentary republic has turned authoritarian, in fact only Sri Lanka in the 70s has done so since world war 2.
Also I take issue with this stawnt as basically no Presidential Republic that had a functioning Presidential Republic prior to WW2 has gone authoritarian. What your pointing to has more to do with the overall legitimacy of governments themselves.
In addition your claim itself is false as we have plenty of parliamentary republics that devolve into authoritarian dictatorships including but not limited to - Mynamar - Nigeria - Ghana - Uganda - South Korea (Second Republic) - Syria - Pakistan experiencing various military coups.